The development of rare earth-cobalt permanent magnets during the decade of the 1970s has enabled marked improvements to take place in such diverse products as electric motors for miniature alarm clocks, stepping motors, a wide variety of switches such as microswitches, lock-in switches, temperature-actuated switches, reed relays, resonant relays, as well as headphones, magnetic bearings, magnetic clamp systems, etc.
The very compact high field density structure provided by these magnets makes them attractive for use in industrial motors. Where alnico magnets have been used in machines of more than about 1 KVA rating which utilize permanent magnets in either the rotor or the stator, or in both, it has been found necessary to supply auxiliary windings on the alnico poles for the calibration of the speed and also for use with a special pulse demagnetizer and remagnetizer for use in case of, for example, bearing failure necessitating the removal of the armature or rotor from the machine. In such machines the need for both permanent magnet and electro-magnetic excitation systems imposes a cost penalty.
The present invention is aimed at providing a motor in which the permanent magnetic flux path through the stator, rotor or both may be momentarily internally shunted so as to prevent it from passing to the rotor of the machine. The rotor can then be separated from the stator and housing for maintenance or repair without magnetic hindrance.